1. Field of the Invention
The present invention relates to a mobile communication system using a hierarchical cell structure (HCS) technology, a cell selection method for selecting a visited cell in such a mobile communication system and a mobile station device therefor.
2. Description of the Related Art
As the demand on and traffic of a radio communication service have increased many technologies for increasing the capacity of a mobile communication system have been used. As one of them, an HCS technology is proposed and used.
For example, a cell structure using the HCS technology in a universal mobile telecommunications system (UMTS) is advised in TR25.304/922 of the third generation partnership project (3GPP) standard, the cell selection logic (at the time of an idle mode and an RRC connected mode) of a mobile station device visiting the HCS structure is described in details. In this specification, the summary of the part of it where the selection logic of user equipment (UE), such as a mobile station device, is defined is briefly described below with reference to this 3GPP standard.
As shown in FIG. 1, the HCS technology is a technology for forming a plurality of cells of cell shapes peculiar to a mobile communication system (a large cell, a small cell, a micro-cell, etc.) into a hierarchical structure. Roughly speaking, it is one object of the HCS technology to eliminate coverage holes and ensure a continuous area by overlapping a small cell on a large cell. As its example in use, it can be considered that by covering an area where traffic is concentrated by a small cell and regarding UE that visits the area as UE that visits the cell (small cell), system capacity can be increased and also a large cell can overlap it.
In the hierarchical structure shown in FIG. 1, the largest cell 11 includes the second largest cell 12 and the cell 12 includes the smallest cells 13 and 14. The cells 11, 12, 13 and 14 are formed by base stations 21, 22, 23 and 24, respectively, and UE 31 and 32 moves in this hierarchical cell structure.
As described above, in an area accompanying a multi-layer structure, it is desired that UE stays in a small cell as much as possible. However, for example, if there are too many numbers of cell selection since the moving speed of UE is too fast and the frequency of reselection is high, UE is made to visit a cell according to its receiving level (large cell) regardless of a cell hierarchy.
Generally, when a hierarchical cell structure is built using a large cell and a small cell, the receiving level of UE also differs due to a difference in the transmitting power of each base station. Generally, since the transmitting power of a base station having a large cell is set larger than that of a station having a small cell, accordingly radio waves from the large cell base station is received more strongly than radio waves from other base stations.
In FIG. 1, although UE in the cell 13 can receive radio waves from the stations 21, 22 and 23, the receiving level of radio waves from the base station 21 is highest, that of radio waves from the base station 22 is the second highest and that of radio waves from the base station 23 is lowest. The same applies to the receiving level in the cell 14.
In such a cell selection logic based on a receiving level, multilayered cells cannot effectively used simply by making a cell structure hierarchical. Therefore, in the HCS technology, waiting control (cell selection control) in an area having a multilayered cell structure can be performed by assigning priority to each cell and notifying UE of the priority information.
By increasing the number of base stations and radio channels for providing a service in a specific geographical area (around a concert hall, the downtown, a station, etc.) and intentionally controlling the connection destination of UE using this technology, the system capacity can be increased.
The above-described priority information can be assigned for each cell as “priority level” and reported from a UMTS terrestrial radio access network (UTRAN) to UE using the system information block of broadcast control channel (BCCH). The detailed information of the system information block of BCCH and the like are described in 3GPP TS25.331 v5.5.0 and after.
FIGS. 2 and 3 show the contents of system information block type 3/4 and FIG. 4 shows the contents of system information block type 11/12. In the idle mode, system information block type 3 or 11 is used.
The parameter 41 (HCS_PRIO) in the element 40 shown in FIG. 3 indicates a priority level and can be set in the numerical range of “0˜7”. According to the specification of 3GPP, the priority level can be defined as “HCS_PRIO” in the element 45 in system information block type 11/12 shown in FIG. 4. The larger the numerical value of the priority level, the higher the priority and the smaller the numerical value, the lower the priority. Generally, the priority of a large cell is set lower than that of a small cell.
In the hierarchical cell structure shown in FIG. 1, the cells 13 and 14 have the same priority, which is set higher than that of the cell 12. The priority of the cell 12 is set higher than that of the cell 11. Therefore, priority level of each base station is, for example, set as follows.
Base stations 23 and 24: 7
Base station 22: 4
Base station 21: 0
However, the relation among transmitting power of the base stations is as follows, as described above.
Base station 21>base station 22>base stations 23 and 24 In other words, the purpose of the priority control is to enable a cell selection according to the priority of each cell regardless of the height of the receiving level. For example, when UE 31 stays in the area of the cell 14, by giving higher priority to the small cell 14, the UE 31 can be made to visit the cell 14 having a low receiving level instead of the large cell 11 having a high receiving level.
It is an object of a mobile communication system having a hierarchical cell structure to provide low-speed/fixed UE with a service in the layer of a small cell. For example, it is preferable that the UE 31 shown in FIG. 1 visits the small cell 14 as much as possible.
However, if a service is provided for UE in the layer of a small cell when UE moves in high speed as when moves in a vehicle and so on, “cell reselection control” in which UE must reselect a visited cell will be repeated many times. In such a case, it is preferable to make it receive a service in the layer of a large cell.
In FIG. 1, a case where UE 32 in a vehicle moves from an area covered by only the large cell 11 to an area also covered by the small cell 14 in high speed is studied below. In this case, when the UE 32 moves between the small cells 13 and 14 in a short time from the relation between moving speed and an area size and when the UE 32 immediately moves to an area covered by only the large cell 11 again, it is desired that it visits the large cell 11.
In order to perform such control based on the moving speed of UE, the present HCS technology provides a mechanism for transferring UE to a large cell and reducing the number of cell reselection if UE moves in high speed even when UE must visit a small cell according to priority information.
In this mechanism, when detecting the moving speed of its own station, UE receives BCCH information (system information block) from UTRAN and obtains necessary information, such as the parameters 43 and 44 (TCRMAX and NCR) shown in FIG. 3 and the like from various pieces of BCCH information. Then, if UE carries out a prescribed number NCR of times or less of “cell reselections” in a specific time TCRMAX, UE determines that its own station is in a slow moving state, selects cells having the highest priority level from cells that satisfies required quality and further determines a large cell having the highest receiving level of them as a transfer target. If UE carries out more times than the prescribed number NCR of “cell reselections” in the specific time TCRMAX, UE determines that its own station is in a fast moving state, selects a cell having the highest receiving level regardless of its priority level and is transferred to the cell from a small cell that has visited. Generally, a cell having the highest receiving level is a cell having a low priority level, that is, a large cell.
FIGS. 5 and 6 is the flowchart of such a cell selecting operation. When the power is switched on (step 51), firstly, cell information stored before the power is switched off is checked (steps 52 and 53). If there is cell information, a cell is searched for on the basis of the information (step 54) and it is checked whether cells having a receiving level more than a prescribed one (threshold level) have been detected (step 55).
If cells having a receiving level more than a threshold level are detected, a cell having the highest receiving level is selected from those cells, receives a system information block (SIB) (step 56) and the initial cell selection is completed (step 57).
If in step 53 there is no cell information, initial cell search control (3GPP TS25.304) is executed (step 64) and it is checked whether cells having a receiving level more than a threshold level have been detected (step 65). If cells having a receiving level more than a threshold level are detected, a cell having the highest receiving level is selected from those cells, a system information block (SIB) is received (step 66) and the initial cell selection is completed (step 57). If in steps 55 and 65 no cell having a receiving level more than a threshold level is detected, the operation in step 64 is performed.
After the initial cell selection is completed, then it is checked whether the received SIB (type 3 or 11) includes HCS information (step 58). This HCS information corresponds to the element 40 shown in FIG. 3 or the element 45 shown in FIG. 4.
If the SIB includes HCS information, its receiving level is measured on the basis of the information (step 59) and the measured receiving level is compared with the threshold value Qhcs included in the SIB (step 60). In the SIB shown in FIG. 3, Qhcs corresponds to the parameter 42 in the element 40 and in the SIB shown in FIG. 4, Qhcs is described in the element 45.
If there are cells whose receiving levels are equal to or more than Qhcs, the priority levels of those cells is checked (step 61) and a cell having the highest priority and the highest receiving level (step 62) is selected. Then, a timer for measuring specific time TCRMAX is activated, the count of the number of times of cell reselections is started (step 63). Then, UE enters a waiting state (step 69).
If in step 58 the SIB includes no HCS information, waiting control (3GPP TS25.304) in a structure other than HCS (step 67) is performed. If in step 60 the receiving level is less than Qhcs too, waiting control (3GPP TS25.304) in a structure other than HCS is performed (step 68).
After UE entering a waiting state, it is checked whether cell reselection has been executed (step 70) and the number of times of cell reselections is counted up every time cell reselection is executed (step 71). Then, the count-up of the number of times of cell reselections is repeated until the timer value reaches TCRMAX (step 72).
When the timer value reaches TCRMAX, the number of times of cell reselections is compared with NCR (step 73). If the number of times of cell reselections is more than NCR, it is determined that its own station is in a fast moving state (step 74). Then, a cell having the highest receiving level is selected regardless of its priority level (step 75), UE enters the waiting state in the cell (step 76) and the operations in step 58 and after are performed.
If the number of times of cell reselections is equal to or less than NCR, it is determined that its own station in a slow moving state (step 77). Then, a cell having the highest priority level is elected, UE enters a waiting state in the cell (step 78) and the operations in step 58 and after are performed.
For example, when building the three-layer cell structure as shown in FIG. 1, it is designed in such a way that any cell may have almost the same number of UE regardless of its size, in a system design concept. However, since according to the present HCS algorism, there are only two ways of fast movement and slow movement as the criteria of cell reselection, only the largest cell 11 (at the time of intermediate/fast movement) or the smallest cell 13 or 14 (priority control at the time of slow movement) can be selected. Therefore, even when UE is desired to visit an intermediate cell 12, it cannot visit the cell 12.
If UE moves in too small a cell in high speed, its battery consumption increases and its continuous waiting time decreases, since cell reselection must be repeated. If UE moves through too large a cell in low speed too, its battery consumption increases since it must communicate with the base station of a large cell at the time of calling and location registration. Furthermore, in that case, since the load of traffic cannot be appropriately distributed, the load of a large cell becomes heavy beyond its design value.
The following Patent Document 1 relates to cell reselection in a mobile communication system having a hierarchical cell structure and Patent Documents 2 and 3 relate to cell reselection in a mobile communication system having an ordinary cell structure.    Patent Document 1: Japanese Translation of PCT International Patent Application Publication No. 2003-534675    Patent Document 2: Japanese Translation of PCT International Patent Application Publication No. 2002-525938    Patent Document 3: Japanese Patent Application Publication No. 2003-070047